The present invention relates to a hot-wire type air flow meter, and more particularly to a hot-wire type air flow meter for an automotive internal combustion engine, which constitutes an intake system of the internal combustion engine, and is adapted to detect and control the flow rate of intake air.
As shown in, for example, Japanese Utility Model Unexamined Publication No. 56-135127 and Japanese Patent Unexamined Publication No. 60-185118, there has been provided a conventional passage structure for a hot-wire type air flow meter for an automotive engine, in which an auxiliary flow path is formed in an intake pipe; a hot-wire element is arranged in the auxiliary flow path; an obstacle or a complicated bent flow path that is long in the axial direction is provided downstream of the hot-wire element for the purpose of protecting the hot-wire element against a backfire or a backblow of the engine and for the purpose of preventing an abnormal output of the hot-wire element caused by the pulsation of the engine. In such a flow meter, since the auxiliary path portion including the hot-wire element is formed in such manner as to exposed to the primary flow, an output error caused by the temperature increase of the flow meter body is small. However, this arrangement requires a long physical length in the axial direction and a large number of mechanical parts which are difficult to mount. Therefore, this arrangement suffers from defects in compactness and cost.
Also, as disclosed in, for example, Japanese Patent Unexamined Publication Nos. 57-23818 and 57-113926, there has been proposed an arrangement in which a hot-wire type air flow meter and a throttle valve means are disposed close to each other in an integral body. In Japanese Patent Unexamined Publication No. 57-23818, the same techniques as those in the foregoing two publications are adopted in the arrangement in which the auxiliary passage within which the hot-wire element is disposed is defined by a straight pipe and is formed in the central portion of the primary passage. However, in the publication '818, there is no protection for the hot-wire element against backfire and backblow of the engine. The throttle valve downstream of the primary flow might serve as a protection means under the condition where it is almost closed, but the throttle valve will have no use as protection means under the full or almost full open condition thereof. Also, in addition to this problem, this arrangement suffers from another problem in which the flow within the auxiliary flow path tends to become unstable in response to the movement of the throttle valve. Japanese Patent Unexamined Publication No. 57-113926 discloses an auxiliary flow path in which a hot-wire element is disposed within a body wall having a large thermal capacity and having no wide relative transfer area, said auxiliary flow path having an L-shape formed by a first flow path parallel to a primary flow and a second flow path perpendicular to the first flow path. With such an arrangement, it is possible to protect the hot-wire element against blowback or backfire of the engine. However, due to the structure of the auxiliary flow path, since air of the primary flow cannot flow around the auxiliary flow path wall, the temperature of the auxiliary flow path wall is highly increased due to the heat generated by the hot-wire element as well as the heat transferred from the engine. As a result, the air within the auxiliary flow path is heated so that the difference in temperature between the air in the auxiliary flow path and the air in the primary flow path is large. Thus, it is impossible to exactly measure the flow rate of the intake air.
The foregoing prior art is silent with respect to the need for reduction of the pipe length between the hot-wire type air flow meter and the throttle valve means. Therefore, the prior art suffers from the problems of increase of pressure loss in the intake passage and of increase of weight and cost of the equipment. Moreover, the prior art encounters the following difficulties: (1) a heat generation of the hot-wire element; (2) a temperature increase of the auxiliary flow path wall around the hot-wire element due to thermal invasion from the outside, that is, an error due to a difference between a temperature of the actual intake air and the temperature of the air flowing through the auxiliary flow path while impinging against the hot-wire element and a temperature compensation element; (3) a countermeasure against a change of the flow rate distribution ratio between the primary flow path and the auxiliary flow path due to the swirl or change of the intake air or the change of flow downstream of the flow meter, even if the constant distribution is intended; a reduction of flow turbulence within the auxiliary flow path, that is, the reduction of the output noises; (4) a protection for the elements against the counterflow due to the backblow or backfire and the pulsation; and (5) a countermeasure against abnormal output.